Treatment of monazite



Feb. 26,1957 v c. DE: RoHDEN r-:r AL 2,783,125

TREATMENT OF MONZITE Filed July 18. 195o TREATMENT F MNAZTE Charles deRohden, Neuiliy-sur-Seine, and Maurice Peltier, Beaumontel, France,assignors to Societe de Produits Chimiques des Terres Rares, Paris,France Application liuly 13, 1950, Serial No. 174,544

Claims priority, application France .lilly 21, 1949 1) Claims. (Cl.zii-44,5)

It has been known fora long time that monazite (a phosphate of rareearths and thorium) may be attacked by caustic soda. Nevertheless theonly treatment used up to the present by industry is to carry out theattack by sulphuric acid, in spite of the drawbacks resulting therefrom.The principal drawbacks are: l

(a) Loss of the phosphoric acid content, in spite of its Value, as aresuit of the complications to which its recuperation leads.

(b) Small solubility of the sulphates obtained, necessitating thepreparation of dilute solutions, and as a result large volumes.

(c) Bad separation of the thorium and rare earths, enforcing largereturns of intermediary fractions.

(d) High costs of maintenance of the plant treating Very corrosivesolutions (sulpho-phosphoric acid).

In spite of the drawbacks of the acid process, the alkaline process hasnot yet become a practical method. This is due to the fact that it cannot give satisfactory results from an industrial point of view, unlessit is carried out under certain technical conditions, which up to thepresent moment have not been determined.

The present invention precisely hasfor its object a method of alkalinetreatment of-monazite which meets this demand.

According to the invention, after an attack of the monazite, at atemperature near the boiling point of the reacting medium, by NaOH in aconcentration comprising between 55 and 70%, and preferably nearly 65%NaOH, the substance is retreated with hot (above 55) water or with amother-liquor recovered from a subsequent operation, to separate, on theone hand, the insoluble hydroxides, and the other, the soluble sodiumphosphate, thev hydroXides thenundergoing a process of separation of therare earths from the thorium anduranium.

-The retreatment of the substance after attack by the NaOH may be doneby hot water used in a quantity larger than 4.5 cubic meters per ton ofmonazite treated so that the dissolution of the phosphate shall besufciently complete, but preferably lower than 8 cubic meters per ton inorder to avoid too high dilutions which would lead to a bad yield in thecrystallization'of`the sodium phosphate. Y

A good proportion is around 5.5 cubic meters of water per ton ofmonazite treated.

There could be used, in preference to pure water for this retreatment, amother liquor coming from the crystallization of the phosphate or from asecond subsequent retreatment of the hydroxides by the hot water.

The alkaline solution containing sodium phosphate isv separated from thehydroXides by filtering or settling; this last-named method appears tobe preferable andwill be advantageously completed by a iiltration step.vThe sodium phosphate is collected by subsequent crystallization.Y

'In this case, the mother-liquor 'of this vcrystallization is asolutionof caustic soda lye at,l about 13 ,0 'grams per. liter,7

4 how theoperationsmay be carried out. which may be concentrated, forinstance 'by' evaporation,

2,783,125 Fatented Feb. 26, 1957 and used again for the initialattacking operation of theV monazite.

The hydroxides remaining after separation of the alkaline solutioncontaining the phosphate are advantageously washed with a dilutesolution of NaOH to ensure a better elimination of the phosphates, themost concentrated wash water resulting from this operation being able tobe used again for the irst retreatment of the substances resulting fromthe initial attack of the monazite by the NaOH.

It is advantageous in order to carry out the rest of the operations ofthe present process with good conditions of yield that the retreatmentsand washings of the hydroxides shall be performed in such a way that theremainder of P205, in relation to the weight of hydroxide shall be lessthan 1% and even preferably around 0.4% or even lower.

Accordingr to the invention, the hydroxides thus prepared are treated,so as to separate the rare earths and the thorium.

The hydroxides are at first treated by a mineral acid such as HC1, H2504or HNOs under conditions permitting the separation of the thorium.

According to a first species of the invention, the dissolution of thewhole of the hydroxides is effected by an acid in excess of thetheoretical quantity, the solution separated from the unattacked residuethen being partly eutralized for instance by admix'ture of rare earthcarbonates, up to a pH of around 4, in order to precipitate the thorium.

According to a second species of the invention, which permits obtainingcomparable results from a point of View of yield and quality and whichis preferable owing to a saving of reagents and the obtaining of a lessvoluminous precipitate, hydroxides are placed in suspension in thewater` and acid is added until the pH shall be equal to about 3.2 sothat %'at least of the rare earths shall be dissolved and then collectedin the form of pure salt the whole of the thorium and uranium whichremain insoluble being separated in the form of a high 'concentrate ofThOz at Sil-95% in relation to the total oxides, containing about 3% ofP205 in relation to the oxide of thorium ThOz accompanied by unattackedore.

The thorium concentrate thus obtained may be puriiied by any knownsuitable means. For instance by oXalate or iiuoride, with or withoutrecuperation of the anion. Nevertheless, according to the invention theprocess may be carried out as follows:

This concentrate is dissolved in hydrochloric or nitric acid in excess,sulphuric acid being excluded; the-solution is separated from theunattacked residuum by filtration and then is treated by sulphuric acidto precipitate the thorium in the form of its sulphate whichcrystallizesa The' mother-liquor of crystallization of the thoriumsulphate is then treated by a potassium saltpotassium sulphate, forinstance, to precipitate the thorium remaining in the form of doublesulphate of potassium and thorium which carries along the rare'earthspresent, butbeing sent back to the iirst operation of separating thethorium by the acids.

i of the description, there has been shown on the accompanying drawing,the flow sheet of a completecycle of' operations according'to theinvention, and there will be described hereinafter, with numberedreferences to th l. Preparation of the monazite.-About one ton of ore Byway of example and to facilitate the understanding is table,

issnel'y ground, so-as to pass through a 200 sieve with a maximum of10%Y oversizeto sieve 300. The product is preferably moistened andtransformed in the cold into a pulp without lumps.

2. Preparation of thev attacking lye-A solution of'- about l ton ofsodium hydroxide is prepared sotthat` its NaOH content shall be around65% at the momentof the attack in the operation (3).

3. Attack of the montaa-The NaOH solution is gradually admixed to themonazite and the temperature is kept to 14C-145 C. during 2 to 5hoursgenerally 3 to 4 hours.

The attacked percentage is usually. from 98 to 100% of the monazite.

4. Relreatment.-(A) In the case- Where the retreatment is made with purewater, about 5.5 cubic meters of water is used at a temperature higherthan 55 and preferably comprised between 70 C. and 80 C.

(B) In the case where a mother-liquor coming from a subsequentdecantation is utilized, about 6.5 cubic meters of mother-liquor maybeused, comprising 5 cubic meters of mother-liquor coming from thedecautation 11 described hereinafter, to which will be profitably added1.5 cubic meters about 130 grams per liter of NaOH coming from therecuperation operation (9) after draining operations (7).

The retreatment is carried out at a temperature higher than 60 C. andpreferably comprised between 70 C. and 85 C.

The advantages of this cycle of mother-liquor are:

(l) A very hot filtration is avoided in the operation (12) hereinafterdescribed, which facilitates the operation and saves the filter cloth.

(2) A more concentrated NaOH lye is obtained on draining operationwithout exaggerating the thickness of the phosphate mixture to bedrained.

5. First domination-The attack with NaOH has transformed the ore into amixture of insoluble hydroxides in suspension in an alkaline solution ofphosphate of sodium. They may be separated by filtration, but it ispreferable to operate by decantation and to decant the alkalinesolution.

6. Crystallzotion of the sodium phosphate-This is carried out simply bycooling.

7. Filtration-The crystals of phosphate are drained of filtered andwashed.

8. Production of the phosphate- About 1.5 tons. of trisodic phosphateNa3PO412H2O is thus produced which corresponds to about 45% of the sodaput into operation for the attack (3) and over 90% of the P205 containedin the ore. These crystals are of commercial quality and. may bedelivered to the trade.

9. Recupemtion of the caustic soda-The mother.- liquor (9) collected atthis filtration is a NaOH lye at about 12C-150 grams per liter NaOH and4 to 5 grams per liter P205 corresponding to 40 to 45% of theNaOH- usedfor the attack. About 3.5 cubic meters are collected. This mother-liquormay be used in soap-making, for the manufacture of bleaching water, etc.

10. Concentration of the mother-liquor of crystallization-Thismother-liquor may, if preferred, be concentrated in by evaporation andused again as attacking caustic soda in (3), which represents a savingof 40 to 45% of the NaOH necessary for this attack.

In this case, the balance sheet of NaOH will be as follows, for 1 ton ofmonazite:

Attack Recuperation Kilos Kilos Fresh NaOH 60G-550 NaOH of the trisodicphosphate 450 NaOH recuperated.. 40G-500 Recuperated NA 45H00 NaOH lost100-150 11. Second decantation.-The'hydroxides may be retreated. asecondtime. This time the operationis carried out at a highertemperature than 25, preferably at 40-50".

After decantation, at about 20% of the initial volume 1.1 to 1.5 cubicmeters of suspension is produced, containing about 700 kilos of oxides.

The mother-liquor coming from this second decantation may be, asaforementioned, used again for the operation of the first retreatment(4).

l2. Fitrazon and washing.--After the second decantation, the filtrationmay be carried out almost cold, without fear of crystallizations. Thesolution is not corrosive for iron.

First of all 'the washing is preferably done with 'a solution of causticsoda, for instance at grams per liter, then with water. The retreatmentsand washings of the hydroxides are thoroughly developed until the P205in relation to the Weight of hydroxide is lower than 1% and preferablyabout 0.4%` or'even less.

A cake is produced with about 60% oxides.

13. Separation ofthe rare earths and thorium.-This operation must bemade according to one of the two following ways:

(A) Procedure by total dissolution The hydroxides are dissolved inhydrochloric, nitric or sulphuric acid in slight excess (for instance5%) on the quantity theroreticallynecessary.

The solution is filtered (if it is desired togetherv with substancesfacilitating thelfiltration) and the thorium is precipitated -by acontrolled addition of carbonate of rare earths vup to a pH close-to 4.The thorium is separated from the solution-of rare earths by filtration.

(B) Procedure byy lextraction Hydrochloric acid is gradually-added tothe hydroxides mixed with water corresponding to. almost the quantitynecessary for forming neutral salts of rare earths.

The operation proceeds for several hours and must be carried outv withprecaution by watching the pH carefully and avoiding an excess of acid:nal pH close to 3.2.

The iron is oxidized preferably in the fer-ric state during thisoperation, either by hydrogen peroxide, or by an hypochlorite, so as toeliminate it from the solution.

14. F iltration.-The solution is filtered, if desired with substancesaiding the filtration. This gives:

(a) Over (generally 98% of therare earths rendered soluble by theattack), in the form of a solution of almostpure chloride (stage 15),particularly free from P205, SO, ThO2, FezOs which are the troublesomeimpurities for the use, of rare earths, at a concentration which mayreach, if desired 350 to 450 grams per liter of oxides, andy which maybe transformed, either in cast chloridel of commercial quality simply byevaporation (stage 16)', or in pure carbonate by precipitation by sodiumcarbonate (stage 17).

(b) The' whole of the thorium attacked, and the uranium in the form of aconcentrate at 9095% of ThOz/ total oxides and about 3% PzOs/ThOz,accompanied (in the case of the B species) by the unattacked ore andimpurities (stage 18).

19. New dissolution- The thorium concentrate is dissolved inhydrochloric (or nitric) acid (excluding sulphuric acid) in excess of50%, for instance, on the theoretical quantity, so as to produce asolution of chloride (or nitrate) at -150 grams. per liter of ThOz. Thissolution will be at 60-'70 as a result of the evolved heat. IftheV orecontains4 silica or impurities difficult to filter, it can be clarifiedby any usual means so as to facilitate its filtration, which'may becarried out in the hot or cold.

20'.' Fltration.-The residua of the attack are sepa-V rated by ltering.Suchresidua are rejected after conventional washing.` They` onlyrepresent 0 to 2% of the monazite used A('residuum 21).

22. Precipitation` and lzratz'on (separation of the thorium.-'I'here isadmixed to the cold solution dilute sulphuric acid, containing about 125kilos of H2804 to crystallize the thorium sulphate, and this is thenltered. This is also produced in (23) in a commercially pure state witha yield of nearly 90%.

24. Precipitation and Jlterz'ng of the rare earths.-The fraction ofthorium present in the mother liquor is recovered by addition of apotassium salt, the sulphate preferably, until the mother-liquorcontains 3() grams per liter of KzSOi. The operation may be done whilecold. The thorium precipitates inthe form of a double sulphate ofpotassium and thorium which carries away with it the rare earthspresent, and leaves the uranium in the motherliquor.

This uranium may be collected by neutralizing the mother-liquor, intheform of a concentrate, accompanied by a little of the rare earths,thorium and other impurities arising from the initial sand, -such as theiron. It is extracted therefrom profitably by treatment with sodiumcarbonate and precipitation by caustic soda.

26. Attack by the soda and filtration- The double sulphate is profitablyattacked by the caustic soda (stage 26) and the hydroxides produced are,after elimination by washing of sodium sulphate, reintroduced into thecycle for instance at stage 13.

This recuperation moreover only affects a small percentage of the oxidestreated.

These inal recuperations (24 and 26) allow the recycling of about l0 to12% of the initial thorium and l to 2% of the initial rare earths.

Having now particularly described and ascertained the nature of our saidinvention, and in what manner the same is to be performed, we declarethat what we claim l. A method of treating monazite in order to recovervaluable products therefrom which comprises finely grinding saidmonazite to pass through a 200 sieve, treating said linely comminutedmonazite with a NaOH solution of 55 to 70 percent concentration at aboutthe boiling point of the solution, retreating the NaOH digested mona-Zite with Water at a temperature above 55 C., separating the mixture ofinsoluble hydroxides containing the rare earth elements, thorium anduranium from the soluble sodium phosphate formed during the treatmentwith the NaOH and the water, Washing the mixture of insoluble hydroxideswith dilute NaOH until the phosphorus pentoxide content in relation tothe weight of the hydroxides is less than l percent, and separating therare earth compounds from the thorium compounds and uranium compounds. v

2. A method of treating monazite in order to recover valuable productstherefrom which comprises finely grinding said monazite to pass througha 200 sieve, treating said finely comminuted monasite with asubstantially equal amount by Weight of a NaOH solution of 55 to 70percent concentration at about the boiling point of the solution,retreating the NaOH digested monazite with water at a temperature above55 C., separating the mixture of insoluble hydroxides containing therare earth elements, thorium and uranium from the soluble sodiumphosphate formed during the treatment with the NaCl-l and the Water,Washing the mixture of insoluble hydroxides with dilute NaOH until thephosphorus pentoxide content in relation to the weight of the hydroxidesis less than 1 percent, and separating the rare earth compounds from thethorium compounds and uranium compounds.

3. A method of treating monazite in order to recover valuable productstherefrom which comprises nely grinding said monazite to pass through a200 sieve, treating said finely comminuted monazite with a NaOH solutionof 55 to 70 percent concentration at about the boiling point of thesolution, retreating the NaOH digested monazite with water at atemperature above 55 C., separating the mixture of insoluble hydroxidescontaining the rare earth elements, thorium and uranium from the solublesodium phosphate formed during the treatment with the NaOH and thewater, washing the mixture of insoluble 'hydroxides with dilute NaOHuntil the phosphorus pentoxide content thereof is less than 1 percent byweight, and forming insoluble compounds of thorium and uranium andsoluble rare earth salts, and separating said insoluble compounds andsoluble salts thus formed.

4. A method of treating monazite in order to recover valuable productstherefrom which comprises finely grinding said monazite to pass througha 20() sieve, treating said nely comminuted monazite with a NaOHsolution of 55 to 70 percent concentration at about the boiling p saidsolution to a pH of about 4, whereby the thorium and uranium compoundsare precipitated and the rare earth metal compounds remain .in solution,separating theprecipitated thorium and uranium compounds from thedissolved rare earth metal compounds, separating the thorium compoundsfrom the uranium compounds and recovering sodium phosphate and compoundsof rare earth elements, thorium and uranium.

5. A methodV of treating monazite in order to recover valuable productstherefrom which comprises finely grinding said monazite to pass througha 200 sieve, treating said finely comminuted monazite with a NaOHsolution of 55 to 70 percent concentration at about the boiling point ofthe solution, retreating the NaOH digested mona- Zite with water at atemperature above 55 C., separating the mixture of insoluble hydroxidescontaining the rare earth elements, thorium and uranium from the solublesodium phosphate formed during the treatment with the NaOH and thewater, washing the mixture of insoluble hydroxides until the phosphoruspentoxide content in relation to the weight of the hydroxides is lessthan l percent, adding an amount of acid selected from the groupconsisting of hydrochloric and nitric acids sufficient to produce a pl'lvalue of about 3.2, whereby soluble rare earth salts are formed and thethorium compounds and uranium compounds remain insoluble, and separatingthe insoluble thorium and uranium compounds from the solution of rareearth salts.

6. A method of treating monazite in order to recover valuable productstherefrom which comprises finely grinding said monazite, treating saidtinely comminuted monazite with a NaOH solution of 55 to 70 percentconcentration at about the boiling point of the solution, retreating theNaCl-l digested monazite with water at a temperature above 55 C.,separating the mixture of insoluble hydroxides containing the rare earthmetals, thorium and uranium from the soluble sodium phosphate formedduring the treatment with the NaOH and the water, washing the mixture ofinsoluble hydroxides until the phosphorus pentoxide content thereof isless than l percent by weight, dissolving the mixture of hydroxides in amineral acid, neutralizing said solution to a pH of about 4 whereby thethorium and uranium compounds are precipitated and the rare earth metalcompounds remain in solution, separating the precipitated thorium anduranium compounds from the dissolved rare earth metal compounds,dissolviug the thorium and uranium compounds in a mineral acid selectedfrom the group consisting of hydrochloric and nitric acid, separatingthe undissolved residue from the solution of thorium and uraniumcompounds, adding dilute sulphuric acid to said solution whereby thoriumprecipitates and crystallizes as the arsaias 7 sulphate salt and uraniumremains in solution, separating the thorium salt from; the uranium insolution, andrecovering sodium phosphate, and compoundsof rare earthmetals, thorium and uranium.

7. A method of treating monazite in order to recover valuable productstherefrom which. comprises finely grinding said monazite, :treating saidiinely comminuted monazite with a NaOH solution of 55 to 70 percentconcentration at about the boiling point of the solution, retreating theNaOH digested monazite with Water at a temperature above 55 C.,separating the mixture of insoluble hydroxides containing the rare earthelements, thorium and uranium from the soluble sodium phosphate formedduring the treatment with the NaOH and the water, Washing the mixture ofinsoluble hydroxides until the phosphorus pentoxide content in relationto the weight of the hydroxides is less than l percent, adding an amountof acid selected from the group consisting of hydrochloric and nitricacids sufficient to produce a pH value of about 3.2, whereby solublerare earth salts are formed and the thorium and uranium compoundsremaininsoluble, sepa- V rating the insoluble ythorium compounds and uraniumfirst by adding a potassium salt thereto, to precipitate the remaining4thorium as the double salt thereof, and to carry along. the'remainingrare earth compounds, and secondly by separating lthe thorium and rareearth compounds so precipitated from the uranium solution.

9. The process as set forth in claim 8, further characterized in thatsaid thorium compounds andrare earth compounds so precipitated aretreated with caustic soda to produce the hydroXides thereof,vand saidhydroxides are then Washed and recycled.

10. The process as set forth in claim 9, further characterized in thatsaid uranium solution from which said thorium compounds .and rare earthcompounds have been separatedis clarified by treating said solution withsodium carbonate,'iiltering said solution, and precipitating the uraniumcompounds from the filtrate.

References Citedin the le of this patent UNITED STATES PATENTS 617,636Buddeus Jan. 10, 1899 1,366,128 McCoy Jan. 18, 1921 2,196,593 MuskatApr. 9, 1940 FOREIGN PATENTS 315,371 Germany oct. 22, 1919 39,105 lndiaMar. 22, 1949 OTHER REFERENCES Lunge, Sulphuric Acid and Alkali, vol. 2,part 2, 3rd

ed. (1909), pages S-870. Pub. by Gurney and Jackson, London.

1. A METHOD OF TREATING MONAZITE INORDER TO RECOVER VALUABLE PRODUCTS THEREFROM WHICH COMPRISES FINELY GRINDING SAID MONAZITE TO PASS THROUGH A 200 SIEVE, TREATING SAID FINELY COMMINUTED MONAZITE WITH A NAOH SOLUTION OF 55 TO 70 PERCENT CONCENTRATION AT ABOUT THE BOILING POINT OF THE SOLUTION, RETREATING THE NAOH DIGESTED MONAZITE WITH WATER AT A TEMPERATURE ABOVE 55*C., SEPARATING THE MIXTURE OF INSOLUBLE HYDROXIDES CONTAINING THE RARE EARTH ELEMENTS, THORIUM AND URANIUM FROM THE SOLUBLE SODIUM PHOSPHATE FORMED DURING THE TREATMENT WITH THE NAOH AND THE WATER, WASHING THE MIXTURE OF INSOLUBLE HYDROXIDES WITH DILUTE NAOH UNTIL THE PHOSPHORUS PENTOXIDE CONTENT IN RELATION TO THE WEIGHT OF THE HYDROXIDES IS LESS THAN 1 PERCENT, AND SEPARATING THE RARE EARTH COMPOUNDS FROM THE THORIUM COMPOUNDS AND URANIUM COMPOUNDS. 